circle <- function(center = c(0,0),diameter = 1, npoints = 100){
	r = diameter / 2
	tt <- seq(0,2*pi,length.out = npoints)
	xx <- center[1] + r * cos(tt)
	yy <- center[2] + r * sin(tt)
	return(data.frame(x = xx, y = yy))
}
